Amplifying system



Dec. 26, 1933.

V. D. LANDON AMPLIFYING SYSTEM Filed July 22, 1927 ATTRNEY APatented Dec. 26, 1933 AMPLIFYING SYSTEM Vernon D. Landon, vEast Pittsburgh, Pa., assigner to Westinghouse Electric & Manufacturing Company, a Ycorporation of Pennsylvania f Application July 22, 1927. serial rvNo. 207,619

-7 Claims.

My invention relates to amplifying systems, and it has particular relation to systems designed for the amplification currents. r v

Radio signals, when received from far distant points, generally have an amplitude that is too small` to be at once rectified and applied to anV indicating device. It is, therefore, customary'V to precedethe rectifier or demodulator by an am- 'V 10. plifying system of .some sort, that of the usual type comprising va plurality of threeeelectrod'e thermioni'c tubes .connected in cascade by cou' pling devices, vsuch as transformers. Fory the most efficient amplification at radio-frequencies,

it isV customary to so tune the coupling transformers between the adjacent vthermionic tubes that they are substantially resonant to the fre-V quency tobe received,.according tothe teaching of the Alexanderson Patent No.jl,l73,079, employing, for this purpose, a plurality of variable condensers, or variable'inductors.

'In accordance with the present tendency of the art toward the simplification of radio appa? ratus, separate controlling devices for the variable tuning elements have been replaced by unicontrol devices arranged to simultaneously alter the electrical :characteristics ofthe said tuning elements. In view, however, of the practically insurmountable difficulties encountered in making identically similar inductors and capacitors on a commercial scale, the simultaneousv tuningof a plurality of amplification stages tov exact resov nance is not capable of economical commercialA attainment, the best results obtained hitherto being only approximations. Y Y

vIt is, accordingly,rone object of my'invention to provide a multistage high-frequency amplifier Y that shall .be capable of .being precisely tuned by a single tuning condenser. v VAnother object of my invention is to Vprovide a high-frequency amplifier that, when properly adjusted, does nottend to spontaneously oscillate. Another object of my invention is to provide,v in a multi-stage amplier of the type first above v referred to, means wherebyy the selectivity thereofshall be equally as good as that obtained through the use of a plurality of tuned stages.l

Anothercobjectv of `my invention is to provide,

in a multi-stageV amplifier, means whereby regeneration is obtained at the frequency to which the single tuning means is momentarily adjusted, with no tendency toward Aregeneration of incoming signals at differing frequencies.

Another object' of my invention isV to provide an amplifying system' that very materiallyv reduces of high-frequency alternating the `eiect ofqstatic interferencewith received signals.V z y f L Another and more specific object of my invention isto provide aneii'icient amplifying system ,wherein other than the usualf three-electrode 60Y tubes .are'employed 1 ll attain these objects by making use of aplurality ofthermionic devices of the type wherein a grid Vis utilized *to .control the now of current between an anode and an electron-.emissive cathode, anda second grid is interposed between the control grid andthe cathodeV in order to reduce` the so-called space-charge g effect. By proper circuit connections, which will later be explained in detaiL'the effect of ra single tunedy 70 circuit is reflectedback intothe preceding stages of a multi-*stage amplifier lemploying tubes of the type referred to, whereby the effect of a .plurality of tuned` circuitsis simulated withoutactuallyvemploying more than the single tuned stage mentioned previously. f f j Among the features that I 4consider character-'- istie of my invention are those set forth Withparticularity in the appended claims. The invenj tion itself, however,both as toits organization 80 and" its method of operation, together with fur-y therA objects .and advantages thereof, will best be understood by reference to the following descrip-j tion, taken in connection withthe accompanying drawing, in which: f

Figure 1 is 'a diagrammatic view illustrating the principle underlying my invention.` I Fig. 2'is-a diagrammatic view of a multi-stage amplifying system comprising a preferred ern-A bodiment of my invention.

`Referring specifically to Figul, va thermionic device 1 comprising a lament 2, a space-charge grid 3, acontrol grid4 andan anode 5, is 'pro-` vided with asourcejof filament potential 6. The

positive pole ofa potential source 7 is connected 95 to the anode by aconductor 8, and an intermediate point on said source is connected tothe spacecharge grid 3 through an inductorl() shunted` by 1 a tuning condenserll.`

AA source 121of alternating vcurrent is connected 100 between the' control grid 4 and the filament 2,

ka resistor 1?V and acondenser 14 being. interposed in the conductor leading from'the-v source l2 tothe grid 4. Aresistor 15,- a condenser 16 and a grid leak 17 are also` connectedbetween 105 the control grid and the filament, shunt tov the source 12. A conductor 18 extends from the space-chargegrid 3 to one plate of a small condenser 20, the other plate of 'which is'connected to the input circuit by arco'nductor'zl. I 1x10 1,479,779, wherein is shown an additional electrode, known as the space-charge grid, interposed between the usual control-grid and the,V

filament, in order to neutralize, to some extent, the space charge which surrounds the iilament and is generally conceded to comprise a vast number o free electrons. When a constant positive potential is impressed ongthe space-charge grid, with respect to the cathode, the impedance between the anode and the cathode is materially lowered, and it has also been observed that the current flowing to the space-charge grid varies oppositely to the current iiowing in the anode circuit when a variable potential is impressed uponthe control grid. That is to say, as the current to the anode'increases, the current to the space-charge grid decreases, and vice versa.

By reason for this phenomenon, since the voltage at the plate', provided the plate load is resistiveV in character, is 180 out of phase with the voltage of the control grid, it is clear that the voltage ofthe space-charge grid willbe in phase with the exciting voltage when its external load is resistive.

By suitably tuning an output circuit associated with the space-'charge grid to the frequency impressed upon the control grid, such output circuit assumes the characteristics oi a pure resistance. If the inductor and condenser which comprise the said output circuitwere entirely free from losses, the resistance of the tuned circuit to the resonant frequency would be infinite.

The more nearly the characteristics or" the output circuit associated with the space-charge grid approach the characteristics of a purev resistance, the more nearly will the voltageV of the space- C charge grid come intol synchronism with the voltage on the control grid. It is, therefore, apparent that, if the tuned output circuit -11 is replaced by a pure resistance, the .conditions of. a tuned circuit will be simulated..

Ii the alternating potential source 12 supplies to the control grid' of the thermionic device l a potential varying at a denite frequency, and

' if' the output circuit 10-11 istuned to this frequency, the voltage on the space-charge grid pwill be in synchronism with that voltage being iinpressedon the vsaid control grid. The impressed voltage causes a current to flow both in the resistor 15 and in the condenser 16 connected in shunt thereto. By suitably adjusting the ca- L pacity of' the condenser 2O connected between the space-charge grid and thecontrol grid, with respect to the voltage developed at the spacecharge grid, it is possible to cause a'current to flow in the said condenser equal in amount and in! direction to the current flowing in the condenser 16 by'reason of the' input excitation. Under suchv circumstances, the reactance of the input condenser is neutralized, and the input circuit, therefore, displays resistive characteristics only.

This system of capacity neutralization may be applied to amplifying devices in order to increase the tuning range thereof. The stray capacity associated with the input circuits of the various thermionic tubesembodied in an amplifier system places a definite limit below which the system cannot be tuned. Such capacities may be cancelled to any desired degree by adjusting the feed-back from the space-charge grid to the control grid, thus increasing the wave-length range at the short-wave end of the scale. The long- Wave end of the scale will be only slightly affected, since a much larger capacity is being employed for tuning and only a very small percentage of it will be cancelled.

Various other uses of capacity-cancellation will suggest themselves and one use in particular which I consider extremely valuable and important is in connection with a multi-stage amplifier.

A. preferred embodiment of my invention that is shown in Fig. 2 comprises a plurality of fourelectrode therinionic devices 31, 32 and 33, with which are associated untuned coupling networks comprising resistors 34 and condensers 35 analogous to the resistorl and the condenser V16 shown in Fig. 1. Each of the four-electrode thermionic devices is, in addition, provided with a capacity-neutralization condenser 36 equivaient to the condenser shown in Fig. 1. The resistor 34 and the condenser 35 at the left of the thermionic device 31 maybe included in an antenna-ground circuit, as illustrated;

The thermionic device 33 is further provided wih a tunable output circuit equivalent to the circuit comprising the inductor 1G and the variable condenser ll, which output circuit is coupled to the input electrodes of a detector tube 37. The tunable output circuit preferably takes the formA of a band-pass lter 38 capable of being tunedV to offer substantially the same impedance to a plurality oi frequency bands, each bandA being wide enough to accommodate the side bands associated with any incoming frequency in the range of frequencies which the set is intended to receive. In its simplest form, the band-pass filter 38 comprises merely an inductor having a tuning condenser connected in shunt thereto, although it may comprise a plurality of elements arranged according to the well-known'teachings i' of slightly more positive potential than a point 5.

thereon to which the space-charge grids of the same devices are connected by means of a conductor 43.

EEO

The plate of the detector tube 37 is connected Y to the voltage source 42 through'the primary 44 oi anaudio-frequency transformer 45, the secondary 46 of which may either be vconnected directly to a sound-producing device, such as a telephone or a loud speaker 47,l or may be connected to succeeding audio-frequency amplifier stages. The laments of all of the thermionic devices are 'supplied from a single source 48, and a source 50 of grid-biasing potential may be utilized to supply a negative bias to the control grids of all of the devices, if such bias is found necessary and desirable.

The space-charge grid of each tube is connected to' the control grid of the succeeding tube through a small grid condenser 5l. The antenna resistor 34 associated with the thermionic device 31 Vsuppliesa path for the application of grid-bias po'- tential to the said device, while the bias potential for the devices 32, 33 and 37 is suppliedv through resistors 52, 53 and 54, respectively. f

When-in operation, the band-pass filter 38is tuned to the frequency of an incoming signal and, consequently, the load in the output circuit of the device 33 becomes purely resistive at such free quency. As explained in connection with the condenser of Fig. 1, the condenser 36 of Fig. 2, connected between the control and spacecharge grids of the thermionic device 33, is adjusted to that value'which causes neutralization of the reactance of the condenser 35 shunting the coupling resistor 34, and the load, therefore, which the said resistor causes inthe space-chargegrid circuit of the thermionic device 32 alsobecornes purely resistive in character.

By a repetition of the same sequence of events,

the reactance of the condenser 35 in the net.-V

work between thermionic devices 3l and 32 is neutralized, as is also the reactance ofthe condenser 35 associated with the resistor 34 in the antenna-ground circuit. v

The amplifier system, accordingly, exhibits the same characteristics that it would exhibit if all of the resistor-capacity networks were replaced by` inductor-capacity networks, in the manner shown in the patent to Alexanderson 1,173,079, which patent discloses the principle of tuned radio-frequency amplification. n

A certain small amount of energy at frequencies different from the frequency to which the band-pass filter, or tuned output circuit of the thermionic device 33, is tuned may leadthrough the amplifier, regardless of the retroactive eifect Y of the said tuning. However, this interference, or

leakage, may be controlled in any desired den ygree by increasing the capacities of the three acteristics of tuned networks. Inasmuch as ther simplest embodiment of my invention necessitates only a single tuned circuit, I have thus eliminated the necessity of closely matching inductors and condensers in order to attain uni-control.

g Although I have illustrated and described only a single embodiment of my invention, the underlyingprinciplesthereof are capable of application to a large number of radio receiving and transmitting devices. It is also feasible to apply my invention to the repeater systems used in wired telegraphy and telephony land in wired tubes being in parallel.

radio, and to the ampliersutilized inconnecf tion with the reception of signals over submarine cables..

It is my 'intention,therefore, thatno limitations lcomprising resistive and reactive elements ,be-

tween certain of said devices, and means for neutralizing the reactive components of said networks at any desired frequency.

2. In combination, a plurality of thermionic devices arranged in cascade, coupling networks com'- prising capacitive elements betweenv certain of said devices, means simulating an electrical filter between other of said devices, and means whereby an alteration of the characteristics of said lfilter may be caused to also alter the capacitive react-n ance of at least one of .said networks.

3. In combination, a plurality of thermionic devices ofthe four-electrodetype comprising aV control grid and a space-charge grid, coupling networks including capacitive elements between cer-A tain of said devices, and means whereby variations in potentialof said space-charge grids are utilized to neutralize. the reactance of said capacitive elements.

4. The combination of a non-resonant circuit. including resistive and capacitive elements,.a foury electrode thermionic device comprising a control grid and a space-charge grid, a tunable output circuitv associated with said space-charge grid,

and means for so associating said output circuit with said non-resonant circuit that the latter is caused to exhibit certain of the characteristics of j resonance at the frequency to which said output circuit is tuned.

5. A system forselecting electrical oscillations of a given frequency'from oscillations diiering therefrom in frequency, comprising a plurality of thermionic amplifying devices connected in cascade by non-resonant circuits comprising mainly resistive and capacitive elements, and means for giving to said non-.resonant circuits the substantially selectivity of resonant'circuits.

6. Aseries of four-electrode vacuum-tube ampliiiers connected in succession, the outer grid of each tube being connected to the innergrid of the neighboring tube by way of a coupling condenser.

7. A series of four-electrode vacuum-tube amplifiers connected in succession, the outer grid of each tube being connected to the inner grid of the neighboring tube by way of a coupling condenser, and the anode-cathode circuits of all said VERNON D. LANDON. 

